1. Field of the Invention
This invention relates to an apparatus for counterbalancing, for instance, the weight of the optical head unit in an optical instrument such as a microscope.
2. Description of the Prior Art
Precision optical instruments which require the movement of one member, for example, optical heads or support stages, relative to a fixed member, have utilized various mechanisms to attempt to counterbalance the weight of the movable member. It is mandatory in precision instruments, such as microscopes, that a minimum amount of torque be required to rotate the adjusting knobs which move the movable member throughout the entire range of travel of the movable member.
One mechanism utilized to achieve counterbalance is a mass equal to the weight of the movable member which is suspended on one end of a cable which passes over a fixed pulley. The opposite end of the cable is fixed to the movable member. Obviously, as the movable member is moved up the mass moves down, and vice versa. This mechanism is objectionable as it adds considerable weight to the precision instrument itself, adds significant extra inertia or mass to be moved and requires a substantially larger package to enclose the additional mass.
In recognizing the shortcomings of the above system, several methods have been contemplated which utilize springs of various designs to apply forces, either directly or indirectly, to the movable member to aid in compensating for its weight. For instance, a simple extension coil spring may be suspended from the fixed member by one end while its opposite end is securely attached to the movable member. The spring is calculated to be strong enough to support the weight of the movable member at the point of mid-travel. Moving the member in one direction tends to stretch the spring, while movement in the opposite direction relaxes the spring. Therefore, the spring tends to return the movable member to the neutral mid-travel position. However, in order to keep the movable member in any predetermined position along its travel it is necessary that an additional mechanism must be designed into the apparatus to maintain the position of the movable member and prevent it from returning to the neutral position. Typically, some type of clutch or brake mechanism has been added to the adjusting shaft(s) to counteract the spring force. It will be appreciated that a compression spring mounted below the movable member will act similarly to the extension spring, and therefore have similar problems.
Other attempts at counterbalancing the weight of the movable member have included the use of a torsion spring to act on the drive shaft utilized to urge the movable member up and down. Torsion springs suffer from the same inherent problems as the coil springs described above, as the system is counterbalanced in only the mid-travel position.
Additionally, very long coil springs were tried and, although they were somewhat more acceptable they are costly and require additional space.
In U.S. Ser. No. 650,756 entitled "Cable Drive Focussing Mechanism for Optical Instrument" filed Sept. 14, 1984 for inventors James A. Clark and Michael H. Dobner there is shown a microscope which utilizes a closed loop cable drive mechanism which includes an adjustable torsion spring mechanism for counteracting the forces exerted on the adjusting mechanism by the weight of the movable member. The torsion spring is fixed at one end to the microscope support column while its opposite end is fixed to a collar on the adjusting shaft. The cable is wrapped several times around a wrap reel which is adjustably fixed to the adjusting shaft. By loosening the wrap reel and holding it stationary while rotating the shaft, the torque exerted by the torsion spring may be increased or decreased to compensate for the variations in weight of the stage assembly. However, the torsion spring still only balances the stage assembly at approximately mid-travel and, therefore, requires some type of clutch mechanism as described above. Additionally, travel in either direction beyond mid-point requires either greater or lesser amounts of torque be applied to the adjusting shaft.
The counterbalancing apparatus detailed hereinafter provides a combination of eccentric members and coil springs, to simply but quite effectively, equalize the torque required to rotate the adjusting shaft in a precision instrument, such as a microscope.